Engine compression vs leak down test expert needed - Page 2

Rage · 01-16-2009, 11:49 AM

Quote:

Originally Posted by HotPursuit

How many hrs on the engine?? which 2 cylinders were low?

#1 is 125 psi, #3 is 135 psi, the rest are ~150 psi. Engine rebuilt 36 engine hours ago and all cylinders were ~150 psi after a 3 hour engine breakin. The 36 hours mostly cruising 2500 rpm and wake boarding 2200 rpms.

Rage · 01-16-2009, 12:07 PM

Quote:

Originally Posted by articfriends

You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty

See my response to HotPursuit.

Yes that is why I am seeking help to figure this out.

So far the plausible explainations offered have been that there has been a serious deteriation in the ring sealing below TDC on the #1 and #3 cylinders (such as a wrist pin coming out and scaring the lower cylinder wall, loss of ring seal do to gas wash down) or that something in the valve train has deteriorated (cam lobe, lifter, rocker arm loosening) that reduced the intake valve open period during intake stroke.

What do you think about these as potential causes of the low compression cylinders?

stevesxm · 01-16-2009, 04:31 PM

i'm not trying to be a wiseass here but what i think is this...

if you are to the point now where you actually believe something is wrong, then don't try to talk yourself out of it.

take it out. then take it apart to the point where you can actually confirm or deny your suspicions. if it is on the bench you can pop the rockers off, lock that cylinder 1/2 way down the bore and re do the leak down with the piston mid way . that alone will tell you a lot. you can run a borescope down there and look around. you can dial indicate the lift on the cam.

there was a saying that i had taped to my tool box. it said " never check anything you don't intend to fix" i think that applies here. if something MADE you run these tests, then that intuition shouldn't be ignored. on the other hand if you just did it for something to do but really don't want to go thru the effort and expense to find out if you were right, well then just put the plugs back in and go home early.

it gets to a point with all of these , what doctors call " not remarkable " diagnostic results that you simply have to make up your mind what you want to believe. either say to yourself
" i believe the leak down numbers" and go drive the boat for another 50 hours and check it again. or say to yourself " i really believe the compression numbers and don't want to take the risk " and then take it apart.

i really believe that there isn't any middle ground at this point.

Rage · 01-16-2009, 08:50 PM

Quote:

Originally Posted by ;2779371

i'm not trying to be a wiseass here but what i think is this...

if you are to the point now where you actually believe something is wrong, then don't try to talk yourself out of it.

take it out. then take it apart to the point where you can actually confirm or deny your suspicions. if it is on the bench you can pop the rockers off, lock that cylinder 1/2 way down the bore and re do the leak down with the piston mid way . that alone will tell you a lot. you can run a borescope down there and look around. you can dial indicate the lift on the cam.

there was a saying that i had taped to my tool box. it said " never check anything you don't intend to fix" i think that applies here. if something MADE you run these tests, then that intuition shouldn't be ignored. on the other hand if you just did it for something to do but really don't want to go thru the effort and expense to find out if you were right, well then just put the plugs back in and go home early.

it gets to a point with all of these , what doctors call " not remarkable " diagnostic results that you simply have to make up your mind what you want to believe. either say to yourself
" i believe the leak down numbers" and go drive the boat for another 50 hours and check it again. or say to yourself " i really believe the compression numbers and don't want to take the risk " and then take it apart.

i really believe that there isn't any middle ground at this point.

i used to have a saying taped on my tool box. it said " never check anything you don't intend to fix. "
i think that applies here. if something you noticed made you run these tests then that intuition shouldn't be ignored. on the other hand if you just ran these tests for something to do then just put the plugs back in it and go home early and asssume everything will be fine.

Thanks stevesxm,

The drop in compression concerns me. If it needs to be taken appart to fix something important I will surely do that. But I just put it all back together and am not looking forward to tearing it all down again if not necessary. So I wish to be as sure as possible (with everyone's help who are willing) as to what is wrong so I can take the appropriate action to fix it if it needs fixing rather than to just tear it all totally back down only to find that it is something simple to fix like the rockers are out of adjustment.

Bill

Rage · 01-17-2009, 09:05 AM

Stevesxm,

Where does one get ahold of a borescope suitable to look down in a engine cylinder through the spark plug hole?

MDGperformance · 01-17-2009, 09:36 AM

Really need to run the engine to get accurate compression results,I have checked cold engines(a couple cylinders a little low)then have ran engine and copmpression came up,with the leakdown ok this might be whats going on,snap-on makes a nice camera-scope that you can look down the cylinders,check for scoring

rssteiny · 01-17-2009, 11:01 AM

running a leak down test on engines that have not run since september and its in a garage but no heat and cold here. will it be acurate or do they need to be run first (that i cant do do to being winterized) or can i just crank them over a few times then do it.

Rage · 01-17-2009, 12:28 PM

Quote:

Originally Posted by MDGperformance

Really need to run the engine to get accurate compression results,I have checked cold engines(a couple cylinders a little low)then have ran engine and copmpression came up,with the leakdown ok this might be whats going on,snap-on makes a nice camera-scope that you can look down the cylinders,check for scoring

MDGperformance,

I will check compression on all after engine is warmed to see what the numbers are then. Thanks for the bore scope source suggestion.

stevesxm · 01-17-2009, 01:13 PM

he's right. but just google it. a million people make them and ill bet there are some on e bay cheap.

I'CE · 01-17-2009, 02:39 PM

Quote:

Originally Posted by articfriends

You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty

Smitty, maybe, maybe not, some NASCAR guys used to try and cheat by line honing a block and effectively raise the compression in an engine just a few points SOOOOOOOO yea tech.possible. ie: raising the center line of the crank in the block.